1. Field of the Invention
The invention relates to a process for forming a zinc containing phosphate conversion coating layer on an active metal surface, more particularly a surface selected from the group consisting of (i) steel and other non-passivating ferrous alloys that contain at least 50% by weight of iron, (ii) galvanized steel, and (iii) other surfaces of zinc or its alloys that contain at least 50% by weight of zinc.
2. Statement of Related Art
It is well known that zinc phosphate conversion coating processes produce a solid byproduct called "sludge" in addition to the desired solid conversion coating on the metal being phosphated. In order to continue using a liquid conversion coating composition, sludge eventually has to be removed from the bath and disposed of in an approved landfill site. Sludge reduction is of interest because the number of available landfill sites for disposal of this byproduct is dwindling and known recycling alternatives through chemical treatment are not economical at this time.
A phosphate species that is insoluble, is almost always generated in the phosphating of any iron containing material, even if the principal surface that is conversion coated is zinc, and is most likely be found in sludge, is FePO.sub.4. However, when sludge from zinc phosphating of steel or galvanized steel is analyzed, it is most often found to contain zinc and iron in a 1:3 ratio, indicating that there are other components that also precipitate during the operation. Sludge is generated through three main pathways: Zinc dihydrogen phosphate, the zinc phosphate species with which most zinc phosphating liquid compositions are most nearly at equilibrium, is less soluble at higher temperatures than at lower temperatures, so that some sludge may form during the heating of the composition. The solubility of zinc dihydrogen phosphate is also pH dependent. As a result, some sludge will also form during the neutralization of the bath necessary to maintain the optimum free acid value during continued use of a composition. The third, and unavoidable, source of sludge when treating iron, stems from the reactions that produce the phosphate conversion coating itself.
A typical zinc phosphating bath includes phosphate ions, divalent metal ions, hydrogen ions, and an oxidizing compound such as nitrite or chlorate as the process accelerator. The mechanism of the reaction involves acid attack on the substrate metal, iron in this instance, at micro anodes and deposition of phosphate crystals at micro cathodes. It also involves the liberation of hydrogen and the formation of phosphate sludge. Changes in accelerator can affect the amount of sludge formed, but in general no completely satisfactory theoretical analysis for predicting the amount of sludge under a wide variety of operating conditions has been known.